Robust analytic continuation of Green's functions via projection, pole estimation, and semidefinite relaxation

TL;DR

This paper introduces PES, a robust method for analytic continuation of Green's functions that leverages the causal structure to improve accuracy and noise robustness, especially for spectra with sharp features.

Contribution

The paper presents PES, a novel three-step procedure combining projection, pole estimation, and semidefinite relaxation, outperforming existing methods in noisy environments and extending to bosonic functions.

Findings

PES outperforms Nevanlinna and Carathéodory methods under noise.

Causal projection enhances existing continuation methods.

Method applicable to both fermionic and bosonic response functions.

Abstract

Green's functions of fermions are described by matrix-valued Herglotz-Nevanlinna functions. Since analytic continuation is fundamentally an ill-posed problem, the causal space described by the matrix-valued Herglotz-Nevanlinna structure can be instrumental in improving the accuracy and in enhancing the robustness with respect to noise. We demonstrate a three-pronged procedure for robust analytic continuation called PES: (1) Projection of data to the causal space. (2) Estimation of pole locations. (3) Semidefinite relaxation within the causal space. We compare the performance of PES with the recently developed Nevanlinna and Carath\'{e}odory continuation methods and find that PES is more robust in the presence of noise and does not require the usage of extended precision arithmetics. We also demonstrate that a causal projection improves the performance of the Nevanlinna and Carath\'{e}odory methods. The PES method is generalized to bosonic response functions, for which the Nevanlinna and Carath\'{e}odory continuation methods have not yet been developed. It is particularly useful for studying spectra with sharp features, as they occur in the study of molecules and band structures in solids.